Schedule Today: Jan. 28 (Mon) Jan. 30 (Wed) Next Week Assignments !! SQL Queries. Read Sections 6.1-6.2. Subqueries, Grouping and Aggregation. Read Sections 6.3-6.4. Jan. 30 (Wed) Modifications, Schemas, Views. Read Sections 6.5-6.7. Next Week Constraints Read Sections 7.1-7.3, 7.4.1 for Monday Triggers. Read Sections 7.4 for Wednesday Assignments !! SQL Queries, Subqueries, Grouping and Aggregation. Assignment 2 due Wed Jan 30 Assignment 3, 4 due Monday Feb 4 PDA 3 due Thursday January 31, 5pm. (Friday 5pm OK) Mid Term Exam distributed next week, due Friday Feb. 8 5pm? Winter 2002 Judy Cushing

SQL Queries Principal form: Running example relation schema: SELECT desired attributes FROM tuple variables –– range over relations WHERE condition about tuple variables; Running example relation schema: Beers(name, manf) Bars(name, addr, license) Drinkers(name, addr, phone) Likes(drinker, beer) Sells(bar, beer, price) Frequents(drinker, bar) Winter 2002 Judy Cushing

Example What beers are made by Anheuser-Busch? Beers(name, manf) SELECT name FROM Beers WHERE manf = 'Anheuser-Busch'; Note: single quotes for strings. name Bud Bud Lite Michelob Winter 2002 Judy Cushing

Formal Semantics of Single-Relation SQL Query Start with the relation in the FROM clause. Apply (bag) , using condition in WHERE clause. Apply (extended, bag)  using attributes in SELECT clause. Equivalent Operational Semantics Imagine a tuple variable ranging over all tuples of the relation. For each tuple: Check if it satisfies the WHERE clause. Print the values of terms in SELECT, if so. Winter 2002 Judy Cushing

Star as List of All Attributes Beers(name, manf) SELECT * FROM Beers WHERE manf = 'Anheuser-Busch'; name manf Bud Anheuser-Busch Bud Lite Anheuser-Busch Michelob Anheuser-Busch Winter 2002 Judy Cushing

Renaming columns beer Beers(name, manf) SELECT name AS beer FROM Beers WHERE manf = 'Anheuser-Busch'; beer Bud Bud Lite Michelob Winter 2002 Judy Cushing

Expressions as Values in Columns Sells(bar, beer, price) SELECT bar, beer, price*120 AS priceInYen FROM Sells; bar beer priceInYen Joe’s Bud 300 Sue’s Miller 360 … … … Note: no WHERE clause is OK. Winter 2002 Judy Cushing

Trick: If you want an answer with a particular string in each row, use that constant as an expression. Likes(drinker, beer) SELECT drinker, 'likes Bud' AS whoLikesBud FROM Likes WHERE beer = 'Bud'; drinker whoLikesBud Sally likes Bud Fred likes Bud … … Winter 2002 Judy Cushing

Example Find the price Joe's Bar charges for Bud. Sells(bar, beer, price) SELECT price FROM Sells WHERE bar = 'Joe''s Bar' AND beer = 'Bud'; Note: two single-quotes in a character string represent one single quote. Conditions in WHERE clause can use logical operators AND, OR, NOT and parentheses in the usual way. Remember: SQL is case insensitive. Keywords like SELECT or AND can be written upper/lower case as you like. Only inside quoted strings does case matter. Winter 2002 Judy Cushing

Patterns Example % stands for any string. _ stands for any one character. “Attribute LIKE pattern” is a condition that is true if the string value of the attribute matches the pattern. Also NOT LIKE for negation. Example Find drinkers whose phone has exchange 555. Drinkers(name, addr, phone) SELECT name FROM Drinkers WHERE phone LIKE '%555-_ _ _ _’; Note patterns must be quoted, like strings. Winter 2002 Judy Cushing

Comparing Nulls to Values In place of a value in a tuple's component. Interpretation is not exactly “missing value.” There could be many reasons why no value is present, e.g., “value inappropriate.” Comparing Nulls to Values 3rd truth value UNKNOWN. A query only produces tuples if the WHERE-condition evaluates to TRUE (UNKNOWN is not sufficient). Winter 2002 Judy Cushing

Example bar beer price Joe's bar Bud NULL SELECT bar FROM Sells WHERE price < 2.00 OR price >= 2.00; UNKNOWN UNKNOWN UNKNOWN Joe's Bar is not produced, even though the WHERE condition is a tautology. Winter 2002 Judy Cushing

3-Valued Logic Some Key Laws Fail to Hold Think of true = 1; false = 0, and unknown = 1/2. Then: AND = min. OR = max. NOT(x) = 1 – x. Some Key Laws Fail to Hold Example: Law of the excluded middle, i.e., p OR NOT p = TRUE For 3-valued logic: if p = unknown, then left side = max(1/2,(1–1/2)) = 1/2 ≠ 1. Like bag algebra, there is no way known to make 3-valued logic conform to all the laws we expect for sets/2-valued logic, respectively. Winter 2002 Judy Cushing

Testing for NULL The condition value = NULL always evaluates to UNKNOWN, even if the value is NULL! Use value IS NULL or value IS NOT NULL instead. Winter 2002 Judy Cushing

Multi-relation Queries List of relations in FROM clause. Relation-dot-attribute disambiguates attributes from several relations. Example Find the beers that the frequenters of Joe's Bar like. Likes(drinker, beer) Frequents(drinker, bar) SELECT beer FROM Frequents, Likes WHERE bar = 'Joe''s Bar' AND Frequents.drinker = Likes.drinker; Winter 2002 Judy Cushing

Formal Semantics of Multi-relation Queries Same as for single relation, but start with the product of all the relations mentioned in the FROM clause. Operational Semantics Consider a tuple variable for each relation in the FROM. Imagine these tuple variables each pointing to a tuple of their relation, in all combinations (e.g., nested loops). If the current assignment of tuple-variables to tuples makes the WHERE true, then output the attributes of the SELECT. Winter 2002 Judy Cushing

Winter 2002 Judy Cushing

Explicit Tuple Variables Sometimes we need to refer to two or more copies of a relation. Use tuple variables as aliases of the relations. Example Find pairs of beers by the same manufacturer. Beers(name, manf) SELECT b1.name, b2.name FROM Beers b1, Beers b2 WHERE b1.manf = b2.manf AND b1.name < b2.name; SQL permits AS between relation and its tuple variable; Oracle does not. Note that b1.name < b2.name is needed to avoid producing (Bud, Bud) and to avoid producing a pair in both orders. Winter 2002 Judy Cushing

Simplest Case: Subquery Returns a Single, Unary Tuple Subqueries Result of a select-from-where query can be used in the where-clause of another query. Simplest Case: Subquery Returns a Single, Unary Tuple Find bars that serve Miller at the same price Joe charges for Bud. Sells(bar, beer, price) SELECT bar FROM Sells WHERE beer = 'Miller' AND price = (SELECT price WHERE bar = 'Joe''s Bar' AND beer = 'Bud'); Notice the scoping rule: an attribute refers to the most closely nested relation with that attribute. Parentheses around subquery are essential. Winter 2002 Judy Cushing

The IN Operator Example “Tuple IN relation” is true iff the tuple is in the relation. Example Find the name and manufacturer of beers that Fred likes. Beers(name, manf) Likes(drinker, beer) SELECT * FROM Beers WHERE name IN (SELECT beer FROM Likes WHERE drinker = 'Fred’); Also: NOT IN. Winter 2002 Judy Cushing

EXISTS “EXISTS(relation)” is true iff the relation is nonempty. Example Find the beers that are the unique beer by their manufacturer. Beers(name, manf) SELECT name FROM Beers b1 WHERE NOT EXISTS (SELECT * FROM Beers WHERE manf = b1.manf AND name <> b1.name); Note scoping rule: to refer to outer Beers in the inner subquery, we need to give the outer a tuple variable, b1 in this example. A subquery that refers to values from a surrounding query is called a correlated subquery. Winter 2002 Judy Cushing

Quantifiers Example Class Problem ANY and ALL behave as existential and universal quantifiers, respectively. Beware: in common parlance, “any” and “all” seem to be synonyms, e.g., “I am fatter than any of you” vs. “I am fatter than all of you.” But in SQL: Example Find the beer(s) sold for the highest price. Sells(bar, beer, price) SELECT beer FROM Sells WHERE price >= ALL( SELECT price FROM Sells); Class Problem Find the beer(s) not sold for the lowest price. Winter 2002 Judy Cushing